An ongoing need exists to provide personal-care compositions that prevent damage to the skin and other keratinous tissue from harmful ultraviolet radiation (UV). Much attention has been directed to improving the UV blocking efficacy of personal-care compositions. Most of this work is focused on improving the efficacy and stability of the sunscreen actives within the compositions. While this work has resulted in significant improvements in protection, there are other factors that impact the UV blocking efficacy of personal-care compositions. To provide adequate protection, these personal-care compositions must be applied regularly and in a requisite amount. Therefore, a need exists for personal-care compositions that encourage frequent and ample use, such as by having a pleasant feel without being oily or unstable.
The majority of highly effective sunscreens are oily or oil-soluble. These sunscreens are necessary for UV blocking, but have an unpleasant heavy, oily skin feel and present formulation difficulties such as product stability and compatibility with other components. To counter the heavy, oily skin feel associated with sunscreen compositions, such products are commonly formulated as emulsions. The majority of such emulsions are oil-in-water emulsions wherein the aqueous phase (which in most cases is predominantly water) is thickened with polymeric thickeners. Considerably fewer sunscreen compositions in the form of an inverse emulsion (water-in-oil) exist. Inverse emulsions have a tendency to feel greasy and heavy when the external phase is built using a traditional gel network structure comprising, e.g., mineral oil and fatty alcohols. Alternatively, if silicone-based materials are selected as the primary oil-phase components, other issues become critical, including sunscreen solubility and product stability. It is challenging to thicken the oil-continuous phase with commercially-available materials and still provide consumer-required skin feel.
One way to thicken the composition and provide better skin feel is to use silicone elastomers. However, silicone elastomers are expensive and inefficient. Silicone elastomers typically collapse in the presence of polar sunscreen oils, which ultimately results in product instability in the form of syneresis. This destabilization imposes limits on the amount of polar oil that may be included within the product. In cases where the polar oil is a sunscreen, the product may then have reduced or limited UV protection benefits. There is a desire to develop a structured chassis so that even at high concentrations of polar oil, syneresis does not occur in water-in-oil emulsions.
While it may be possible to thicken and stabilize the oil phase using various types of waxy materials, the resulting emulsions are often aesthetically disadvantageous and/or unstable at higher temperatures. Typically water-in-oil emulsions formulated with wax are solids at room temperature. Consumers find it aesthetically undesirable to use a solid stick on the face and neck. Additionally, at high temperatures, a significant portion of the wax may melt, leading to changes in product viscosity/rheology that compromise the product's composition or aesthetic properties—for example, syneresis, component settling, or substantial viscosity growth.
Therefore, a need exists for a personal-care composition in the form of a water-in-oil emulsion providing ample UV protection and the desirable skin feel attributes of silicone elastomers while maintaining suitable viscosity and stability.